1. Field of the Invention
The present invention relates to antennas for two-way communication, such as radio equipment in vehicles and mobile telephones, and more particularly to planar antennas for such applications.
2. Description of the Related Art
Computers, data terminals, and other electronic equipment in vehicles, such as police cars, employ radios to exchange data and other information with base stations. For example, cellular telephone networks and WIFI Internet connections are commonly used for communication with such mobile equipment. The radio system that links the mobile electronic equipment to the telephone network or the Internet has an antenna on the exterior of the vehicle to send and receive the radio frequency signals. Cellular telephones transmit in the 824 to 845 MHz frequency band and receive signals in the 870 to 896 MHz frequency band. PCS telephones operate in the 1850 to 1990 MHz. frequency band. The WIFI protocol enables communication over different frequency bands, for example the 2.4 GHz ISM band and the 5.0 GHz U-NII band. An antenna that is tuned to operate with one of these frequency bands is not optimum for communication in another frequency band.
A typical communication antenna for a motor vehicle is attached to the exterior surface of the roof or trunk and comprises a short section of rigid wire extending vertically. Separate antennas typically are required in order to communicate on multiple frequency bands. Even though such antennas are relatively short, protruding about one foot from the surface of the vehicle, they are subject to accidental breakage, such as in automatic car washes, and acts of vandalism. These antennas are often considered to be unsightly and a detraction from the aesthetic appearance of the vehicle.
U.S. Pat. No. 5,041,838 describes a low profile, flat disk-shaped antenna for bidirectional communication, such as cellular telephones. This antenna is attached to a horizontal exterior surface of the motor vehicle, such as the roof. A coaxial cable extends through a hole in that surface, coupling the external antenna to the transceiver inside the motor vehicle. This antenna is tuned to a single frequency band.
U.S. Pat. No. 6,087,990 discloses a low profile, flat disk-shaped antenna assembly that combines two antennas into a single package. One antenna is tuned for bidirectional communication equipment, such as cellular telephones, while the other antenna in designed for another type of radio frequency equipment, such as a global positioning satellite receiver. Separate coaxial cables for each type of equipment connect to this dual antenna assembly.
U.S. Pat. No. 6,850,191 describes an antenna assembly has a pair of disk shaped antennas, each tuned to a different frequency band, thereby enabling the same assembly to be used with two different of communication apparatus. One antenna disk lies on top of the other in electrical contact. A single coaxial cable carries the signals for both antennas with one conductor of the cable attached to one antenna and the other conductor is attached to the other antenna.